Vehicle theft has seen a dramatic rise in the last ten to fifteen years due to the sharp increases of price in automobiles and the high cost of parts therefor. There, of course, have been many devices devised and sold to the public in an attempt to thwart auto theft. However, most of these are relatively east to circumvent. As such, it has generally been the policy of a vehicle owner to merely insure the vehicle and buy a new vehicle with the insurance money in the event of a theft. This, of course, is not an acceptable position by the insurance companies and insurance rates have subsequently risen in response to this attitude.
One system that has been developed in an attempt to decrease auto theft is an Automatic Vehicle Location (AVL) system which is essentially a triangulation system. In this system, a transmitter is disposed on each vehicle, which transmitter is a polled transmitter. This transmitter has associated therewith a polling receiver that is tuned to a fixed frequency. Each transmitter and its associated polling receiver have associated therewith a unique identification number or address on the system. When a vehicle is stolen, it is only necessary to transmit a signal out on the fixed polling frequency to all of the receivers. All of the receivers then receive this polling command and look for their address. If their address is found on the polling command, this indicates that information must be transmitted from the transmitter. The transmitter typically incorporates a unidirectional antenna that allows it to emit a radio signal in all directions. This radio signal is substantially the carrier and information is modulated onto this carrier. The transmitted information essentially contains identification information. More importantly, however, is the actual signal strength of the carrier which can be received by strategically located receivers about a given location. Only three receivers are required in order to "fix" a given location by conventional triangulation techniques.
One disadvantage to the AVL systems is that with respect to "throughput." Whenever a vehicle disappears, it is necessary to first transmit the polling command for that particular car and then wait for reception of the transmitted signal. Since there is only a single polling channel and since the strategically located receivers only have a single receive frequency different from the polling frequency, only one vehicle can be polled at a given time. If, for example, there were one hundred cars on the system that were stolen at a given time, this would require one hundred commands to be sent out in sequence, with a wait time after each command. There would then be some delay for the triangulation process, at which time the location of the vehicle would then be fixed. The problem, of course, occurs when the vehicle is moving. It can take upwards of twenty minutes to have a polling command pass through the system and then perform the triangulation operation. This is unacceptable for locating moving vehicles in real time.
Additional systems have been proposed for eliminating vehicle theft which operate by disabling the vehicle. The earliest systems of this type, of course, were those that disabled the vehicle in response to a breaking and entering of the vehicle. However, these again can be thwarted. Further, they can also fail. This has lead to user disabled systems which utilize some type of paging system that can send commands to the vehicle to alter the operation thereof and effectively stop the vehicle. These commands can be of the type that slow the vehicle down, they can be the type that turn off the ignition switch, or any type of command that is operable to affect the parameters of the vehicle. One system is that disclosed in U.S. Pat. No. 5,490,200, issued to Snyder et al. on Feb. 6, 1996.
When utilizing paging systems, there are numerous difficulties to deal with. First and foremost, paging systems are designed to transmit messages and not commands. Therefore, the transmission medium is not necessarily amenable to the transmission of commands over the paging frequencies. As such, although the paging systems do offer an attractive advantage in that they have existing infrastructures, they do have some problems that must be dealt with.